Lithium primary batteries each using iron sulfide as a positive electrode active material (hereinafter merely referred to as “lithium primary batteries”) are highly practical because the lithium primary batteries have average discharge voltage of around 1.5 V and are compatible with other 1.5 V-class primary batteries such as manganese batteries and alkaline manganese batteries. A theoretical capacity of iron sulfide used as the positive electrode active material is as high as about 894 mAh/g, and a theoretical capacity of lithium used as a negative electrode active material is as high as about 3863 mAh/g. Thus, the lithium primary batteries are highly practical as high-capacity lightweight primary batteries.
Of the lithium primary batteries, a lithium primary battery including an electrode group formed by spirally winding a positive electrode and a negative electrode with a separator being interposed therebetween and a cylindrical battery case in which the electrode group is accommodated together with a non-aqueous electrolytic solution has a larger area where the positive and negative electrodes face each other. Thus, such a lithium primary battery has excellent discharge properties under a high load.
Iron sulfide used as the positive electrode active material can be industrially synthesized, but naturally exists as pyrite. Thus, if the positive electrode active material is formed by crushing pyrite, a material cost of the positive electrode active material can be decreased.
However, natural ore may contain impurities, or iron sulfide may react with water or air to generate sulfate iron in the course of forming powder of the positive electrode active material by crushing natural ore. In the case where the positive electrode active material contains such impurities, there is a possibility that the impurities are dissolved from the positive electrode in the non-aqueous electrolytic solution, and that, e.g., iron ions move to the negative electrode and are deposited on the negative electrode. As a result, the following disadvantage is caused: the discharge properties are degraded when dendrites of the grown iron penetrate the separator to cause a minor short circuit.
For the foregoing disadvantage, Patent Document 1 describes a technique to reduce or prevent a minor short circuit in such a manner that a pH value for iron sulfide used as a positive electrode active material is adjusted to a predetermined minimum value to lower solubility of impurities and reduce generation of dendrites.
In the case where lithium used as the negative electrode active material is used for the negative electrode as lithium foil, part of the negative electrode may be raptured in the wounded electrode group due to low tensile strength, resulting in degradation of the discharge properties. Moreover, if the raptured negative electrode penetrates the separator, a disadvantage that an internal short circuit occur is caused.
For the foregoing disadvantage, Patent Document 2 describes a technique to reduce or prevent rapture of a negative electrode in such a manner that a lithium alloy formed by alloying lithium with aluminum is used for the negative electrode to increase the tensile strength of lithium alloy foil.
Moreover, in addition to aluminum, Patent Document 3 describes mercury, zinc, and magnesium as metal which can be alloyed with lithium.